1. Field of the Invention
This invention generally relates to batteries and, more particularly, to a sealed battery.
2. Description of the Prior Art
In recent years, a secondary cell or battery such as a lithium battery, a carbon lithium battery or the like is widely used in video tape recorders, clocks and so on.
The batteries of this kind take a sealed structure, but sometimes cause explosions because the electric power generating element enclosed within the battery causes a chemical change to increase the inner pressure.
If a nonaqueous electrolyte battery, normally such as-the lithium battery is overcharged or short-circuited to flow a large current by mistake or the like, the electrolysis solution may be sometimes decomposed to produce a gas. The battery, when filled with this gas, has its inner pressure raised by this gas and may be instantly destroyed.
U.S. Pat. No. 4,943,497 describes a sealed battery that can prevent such an accident.
This proposed sealed battery, as shown in FIG. 1, has an outer can 2 within which an electric power generating element 1 is enclosed, and a lid 3 which is normally used as the positive electrode terminal and mounted at the open end of the can 2 through a ring-shaped gasket 4 so as to seal the lid 4 to the can 2. In addition, a safety valve 5 is provided within this lid 3. This safety valve 5 is made of a metal plate which is easily deformed by the increase of the pressure within the battery. Also, current cutting-off means 6 is actuated by the deformation of the safety valve 5.
FIGS. 2 and 3 are magnified cross-sectional views of a main part of the above battery, showing the normal state and the abnormal state in which the pressure within the battery is increased, respectively. The electric power generating element 1 has, for example, sheets of negative and positive electrode materials and a separator. This separator has an electrolysis solution impregnated therein. The sheets are wound together in a cylindrical shape, gripping the separator therebetween. In addition, ribbon-like lead plates 7 and 8 are drawn out of the negative electrode and positive electrode sheets. One of the lead plates, or the lead plate 8, is connected to the bottom of the outer can 2 (see FIG. 1).
The safety valve 5 is made of a metal plate of a disk shape and formed to be a dish-like disk shape which projects toward the electric power generating element 1, by press molding. This safety valve 5 also has a safety valve projection 9 provided at the center thereof to project toward the electric power generating element 1. The outer peripheral flange portion of the safety valve 5 is bent to wrap or hold, for example, the outer peripheral edge of the lid 3. This outer peripheral flange and the edge of the lid 3 are pressed by caulking against the open end of the outer can 2 through the ring-shaped gasket 4, thereby sealing the outer can 2.
The lid 3 has apertures 10 bored therein for exhausting the gas. However, the outer can 2 is maintained to be hermetically sealed or to be shut off from the outside by the valve 5.
In addition, a cylindrical disk holder 11 is fixedly fitted on the outer periphery of the safety valve 5 to be held.
This cylindrical disk holder 11 has provided therein a flat plate 11a perpendicular to its axis as shown in FIGS. 4 and 5 which are respectively a plan view of the disk holder 11, and a cross-sectional diagram taken along the line V--V in the plan view. A disk 12 is forced to be fitted within the disk holder 11.
The flat plate 11a of the holder 11 and the disk 12 have central apertures 11c and 12c bored, respectively. The safety valve projection 9 of the safety valve 5 is inserted through these apertures 11c and 12c to reach the electric power generating element 1 side of the disk 12. Thus, the tip of this projection 9 hits the positive-electrode lead 7 of the electric power generating element 1. Then, the tip of the projection 9 and the lead 7 are joined by ultrasonic welding or other means.
The disk 12 and the flat plate 11a of the disk holder 11 also have apertures 12W and 11W bored at the corresponding positions, respectively, so that gas can be exhausted through these apertures.
If a gas is generated by some cause in the accommodating portion for the element 1 of the battery to increase the inner pressure, the safety valve 5 is pushed up by the pressure of the gas which has passed through the aperture 12W of the disk 2 and the aperture 11W of the flat plate 11a of the disk holder 11. As a result, the safety valve 5 is deformed, or swollen toward the lid 3 as shown in FIG. 3. Therefore, the pressure within the battery can be reduced by this deformation of the safety valve 5, and also the projection 9 is forced to rise in proportion to the deformation of the safety valve 5. At this time, the ribbon lead 7 welded to the tip of the projection 9 is also forced to rise, but the lead 7 cannot be moved because it is blocked by the disk 12. Thus, the tip of the projection 9 is pulled off from the ribbon-shaped lead 7 so that the lead 7 is electrically disconnected from the safety valve 5, or that the element 1 of the battery is electrically disconnected from the lid 3. The current cutting-off means 6 is thus constructed as above.
In this way, the gas can be stopped from generation and the pressure within the battery can be suppressed from increase.
According to this arrangement, however, the ribbon-shaped lead 7 which is generally about 3 mm wide and flexible may be drawn in the central aperture 12c of the disk 12 when the pressure within the battery is increased to deform the safety valve 5. At this time, the lead 7 is not always be surely pulled off, or cut from the joint with the tip of the projection 9. In addition, since the cut end of the lead 7 which is disconnected and pulled up to float may be inadvertently made in contact with the can 2, or with the negative electrode to cause a short-circuit, thus losing the safety.